Materials such as diamond and cubic silicon carbide, which are characterized by large electron valance band gaps, are intrinsically superior to silicon, germanium, or most of the III-V compounds for high temperature, high performance electronic devices. This is because their wide electron valence-to-conduction band gaps require a large amount of energy input before the electrons will move between the bands. The band gap refers to the energy difference between the uppermost point of the valence band and the lowest point of the conduction band within the band structure for the material. Therefore, these materials can tolerate a great input of thermal energy, resulting from exposure to extremely high temperatures, with a minimal amount of electron movement between the electron bands. This is not the case with silicon, germanium or conventional III-V materials wherein the band gaps are narrow, thereby requiring a significantly smaller amount of thermal energy to force the movement of electrons between bands. Accordingly these conventional materials can not operate effectively in semiconductor applications at elevated temperatures. As an illustrative example, a conventional CMOS type of device can not operate at temperatures over about 250.degree. C.
However, even though diamond and cubic silicon carbide possess this potential advantage for high temperature use, transistors from these materials have not proven feasible. Diamond-based transistors have not been realized in practice because the process of doping the diamond to form an electrically conductive n-type material has proven to be extremely difficult. Furthermore, regardless of the deposition procedure used, diamond films are unsatisfactory for electronic applications because of their polycrystalline structure and their tendency toward rough, multifaceted surfaces. In addition, diamond films cannot be grown epitaxially on any substrates other than diamond and cubic boron nitride, which severely hampers its usefulness.
Although the development of cubic silicon carbide is further along than the development of diamond, problems associated with the final film quality and the doping, etching, and oxidation of the films, are also imposing to device development. As for the silicon carbide, films of this material that are grown on silicon have rather poor properties because of the crystallographic lattice mismatch between the materials. As a result, other substrate materials with better lattice matching parameters are being investigated. Therefore, diamond- and silicon carbide-based electronic devices, although theoretically advantageous, are not likely to replace the current silicon- or germanium- based devices until the properties of these films can be improved substantially.
As an alternative III-V material, Boron Nitride (BN) is particularly interesting from both the practical and scientific viewpoints. Boron nitride is characterized by at least three different crystal structures: hexagonal, wurtzite and cubic zincblende. The boron nitride phase having the cubic zincblende crystal structure is particularly useful since it is characterized by many desirable physical properties which are comparable to diamond, including high electrical resistivity and high thermal conductivity. In addition, the cubic zincblende crystallographic phase of boron nitride is relatively inert chemically.
Because of these properties, this cubic form of boron nitride is potentially very useful as a replacement for diamond in electronic devices. Since the physical properties of cubic boron nitride are comparable to diamond, cubic boron nitride-based devices should be capable (like diamond) of operating at higher temperatures, higher speeds, and higher output powers, as compared to similar devices formed from silicon, germanium or conventional III-V materials.
A bipolar transistor is the most commonly used device in microelectronic applications and is an example of the type of device in which the cubic boron nitride material may be useful. Bipolar transistors are used routinely in integrated circuits as resistors, capacitors and diodes. In addition, bipolar transistors are commonly used as power transistors, high frequency microwave transistors and switching transistors. Although there is no well defined boundary between power transistors and microwave transistors, typically power gain and efficiency are the prime considerations for a power transistor while cutoff frequency and noise are the prime considerations for a microwave transistor which generally performs small-signal amplification. Alternatively, a switching transistor must be able to switch from a high-voltage low-current (on) condition to a low-voltage high current (off) condition over a very short time period. Therefore the most important considerations for a switching transistor are current gain and switching time.
In order to determine a suitable material for use in these types of devices which incorporate bipolar transistors yet which have widely divergent considerations, the Johnson and Keyes figures of merit may be used for evaluating the various types of semiconducting materials. Generally, the Johnson and Keyes figures of merit are commonly used throughout the industry and evaluate a material based on several criteria, including factors such as the dielectric breakdown voltage and the saturated charge carrier velocity of the material. The Johnson portion of this scale evaluates the usefulness of a semiconductor material as a high-power amplifier of microwaves and millimeter-waves. The Keyes portion of the scale evaluates the usefulness of the semiconductor material in dense, integrated circuit applications.
When diamond is evaluated using this scale, the Johnson and Keyes figures respectively indicate that diamond is approximately 8000 and 32 times better than silicon for these various applications. Although some of the physical properties necessary to calculate these Johnson and Keyes figures of merit are unknown for cubic boron nitride, it is expected that the Johnson and Keyes figures for cubic boron nitride are also similar to diamond since the properties of the cubic boron nitride are so similar to that of diamond. Therefore, it would appear to be advantageous to provide a bipolar transistor formed from the cubic boron nitride material.
In addition, the dielectric strength of cubic boron nitride is greater than 10.sup.7 Volts cm.sup.-1, as compared to a dielectric strength of 5.times.10.sup.5 Volts cm.sup.-1 for silicon. This factor of 20 increase in the dielectric strength of the cubic boron nitride would result in a bipolar transistor having a potential power output increase of about 400 times that of a similar silicon-based device. Again, it appears that a bipolar transistor formed from the cubic boron nitride is needed and could be extremely advantageous for these high power device applications.
It is also to be noted that the transistor speed of a semiconductor material is affected by various factors, including the dielectric constant of the material and the charge carrier mobility and velocity, as well as other factors. In particular, the dielectric constant of the material determines the parasitic capacitance of the transmission lines within an integrated circuit, and therefore the transistor speed is inversely related to the dielectric constant of the semiconductor material. Based on the dielectric constants for the various materials, cubic boron nitride transistors have the potential of being 1.7 and 1.8 times faster than silicon and germanium transistors, respectively. Thus, it would be potentially advantageous to use transistors formed from cubic boron nitride for computers or other applications where a higher speed is required.
Also, just as diamond is characterized by a large band gap between electron bands, so is cubic boron nitride. The large band gap of cubic boron nitride, which is greater than 6 electron volts, minimizes the thermally induced and random movement by electrons between valence shells within the elements. This enables a cubic boron nitride device to operate at extremely high temperatures, such as for example 1000.degree. C. or higher. In practice, the upper operating temperatures will be limited primarily by the temperature limitations imposed by the metallization used for making the ohmic and/or Schottky contacts to the cubic boron nitride semiconductors. Therefore it is obvious that cubic boron nitride transistors have the potential to enjoy widespread use in high temperature applications, as well as in a multitude of various other applications.
Unfortunately, conventional methods for growing cubic boron nitride films have been even less successful than the methods used to produce diamond films. Until recently, the most successful of these methods has been ion beam deposition. Boron nitride films grown by this method exhibit the cubic structure and appear to contain only a single phase. However, the films also exhibit the same morphology as diamond films in that they have rough, polycrystalline surfaces.
Recently though, smooth epitaxial films of cubic boron nitride have been successfully grown on silicon wafers by means of a pulsed laser evaporation technique, as disclosed in United States Patent application Ser. No. 07/446,758 to Gary L. Doll et al., entitled "Laser Deposition of Crystalline Boron Nitride Films", filed on Dec. 6, 1989, and assigned to the same assignee of this patent application. With this pulsed laser evaporation method, cubic boron nitride films were epitaxially grown on a silicon substrate oriented along the [100] axis.
Therefore, it would be advantageous to provide a cubic boron nitride bipolar transistor, which may be suitable for use in various electronic applications, including high temperature and high performance semiconductor devices. It would also be advantageous to form such a cubic boron nitride bipolar transistor using pulsed laser evaporation techniques.